Yamaha 20 HP 1997

[dingoboats]

I have a 1997 20 HP yamaha 2cylinder, single carb 2 stroke.
It has decided to run on one cylinder only !
I have substituted the plugs, coil and CDI from another motor and still it isn't fixed. Top cylinder only runs.
Where do I look now ?
Any help would be appreciated .
Thanks

[rodbolt17]

two methods after a spark and compression test.

method 1, use a peak reading multi-meter and the tune up spec guide and test the pulser coil outputs and the CDI outputs.
if the outputs are good and a simple spark test fails you found a coil issue.
if the spark test passes quit looking at ign at all and inspect for mechanical issues such as a broken reed.

method 2, simply stand back and chunk parts and maybe it will go away before you blow a piston.

[99yam40]

are you sure all cylinders are getting spark?
spark tester will show if it is or is not

If it is then, check fuel pump for leaking into crank case

[dingoboats]

are you sure all cylinders are getting spark?
spark tester will show if it is or is not

If it is then, check fuel pump for leaking into crank case

Thanks 99yam40 - removed fuel pump and found tiny split in "mylar" diaphram.Bottom cyl had buckets of fuel in it

Thanks to all who responded - built a little spark tester & checked for both cylinders with spark.Also removed flywheel and checked all wires etc.

Thanks from Down Under.(Australia)